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How long is the cycle of Earth’s axial precession?

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Engraving of six astronomers, with Hipparchus seated at center and, from left to right, Galileo, Johannes Hevelius, Tycho Brahe, Nicolaus Copernicus, and Ptolemy standing, each holding scientific instruments. — Hipparchus “Six astronomers,” the frontispiece of *Astronomica Institutio* (1692) by Jan Luyts, engraving after drawing by Gerard Hoet. Hipparchus is seated at center with from left to right: Galileo, Johannes Hevelius, Tycho Brahe, Nicolaus Copernicus, and Ptolemy.

The cycle of Earth’s axial precession is approximately 25,772 years. This phenomenon, which involves a slow, cyclic wobbling in the orientation of Earth’s axis of rotation, was discovered by the Greek astronomer Hipparchus while he was compiling his star catalog.12